深水柔性立管涡激振动与上部平台运动的动力耦合研究

Since the structural configuration of ocean riser and distribution of fluid field along the riser span are getting more complicated for deep water platforms, the dynamic responses such as vortex-induced vibration (VIV) of slender riser becomes increasingly challenging, especially, if the additional effects introduced by top motion of floating platform are involved. The coupling mechanism and dynamic response behavior of floating top end with marine flexible riser undergoing VIV still need further theoretical development along with experimental observation. In this study, we will focus on the dynamic interaction between floating top body and marine riser suffering VIV. The prediction model for the VIV and dynamic response of a coupling platform-riser system will be presented by means of theoretical analysis combining numerical simulations using finite element approach as well as corresponding model experiments. The theoretical and experimental results are expected to be valuable in both aspects of academic research and practical engineering, e.g. to improve structural endurance and reliability of deep-water platform in the South China Sea. The work addressed in this project is supposed to cover the following 4 areas: 1) the mathematical expressions of dynamic forces for both riser and floating top in lock-in range, along with the coupling dynamics between these two parts, then, the static/dynamic governing equations of the coupled system; 2) different impacts of various top motions such as surge/sway, heave and roll/yaw/pitch on the lock-in region and response amplitude of riser VIV, such as nonlinear amplification and parameter excitation; 3) numerical simulations of nonlinear dynamics response for significantly flexible riser experiencing vortex-induced lift force along with top end motion; 4) for case of unsteady VIV, the remarkable non-dimensional factors representing structure, fluid and coupling system respectivelly, finally, the prediction model specifically for unsteady VIV and the generalization to wider parameter range.

深水立管结构形式和流场速度分布变得复杂,而且由于平台浮体运动幅度增大使得水下立管与上部平台运动的耦合作用更加明显,因此深水柔性立管的涡激振动响应预测更加困难，需要研究水下立管与上部平台的动力耦合机理和动响应规律。本课题利用理论研究结合数值计算和模型试验，进行考虑平台运动影响的深水立管涡激振动研究，建立相应的非定常响应预测模型；研究结果对提高深水平台的结构安全性具有一定的工程应用价值。 主要研究内容：1)锁频阶段立管动力、平台动力及二者的耦合作用，相互作用的静力/动力学描述；2)不同平台运动形式（如纵/横荡、垂荡、纵/横摇和首摇等）对立管涡激振动的锁频范围和振动幅值的不同影响机理和规律，如非线性振幅放大、参数激励等现象。3)大柔度立管在平台运动和涡激升力共同作用下的非线性动响应的数值模拟；4)非定常流中，大柔度立管涡激振动的描述模型、主要影响因素以及参数推广方法。

深水立管结构形式和流场速度分布变得复杂,而且由于平台浮体运动幅度增大使得水下立管与上部平台运动的耦合作用更加明显,因此深水柔性立管的涡激振动响应预测更加困难，需要研究水下立管与上部平台的动力耦合机理和动响应规律。本课题利用理论研究结合数值计算和模型试验，进行考虑平台运动影响的深水立管涡激振动研究，建立相应的非定常响应预测模型；研究结果对提高深水平台的结构安全性具有一定的工程应用价值。.主要研究内容：锁频阶段立管动力、平台动力及二者的耦合作用，相互作用的静力/动力学描述；各种平台运动形式（如纵/横荡、垂荡、纵/横摇和首摇等）对立管涡激振动的锁频区域和响应幅值的不同影响机理以及影响范围；非线性立管在平台运动和涡激升力共同作用下的动响应数值模拟；运动边界对水动力和结构动力的展向相关性与相关长度的影响。.结果表明：1）浮体不同运动形式与水下结构的耦合机理不同。浮体横荡会造成运动边界效应，由于非线性耦合，水下结构的响应出现了振幅放大现象，水面浮体振动会在沿着水下结构传播的过程中被放大；由于该现象的存在，即使在较小的流速下，由于结构和流场的相对速度增大，也会引发新的涡激振动锁频区域；垂荡引起结构参数周期变化，从而导致结构轴向动张力和参数激励，在平台垂荡和涡激振动的共同作用下，结构的动响应会大于涡激振动与参数激励分别单独作用的响应。2）获得了浮体运动幅值、频率对水下细长结构涡激振动的影响规律，揭示了耦合过程中振幅放大、新锁频区域以及动响应中模态转换等新现象。与不考虑浮体运动相比，水下结构的动响应位移增大了多倍，而且振幅放大倍数随着模态阶数的降低而增大；浮体横荡振幅越大，水下结构振动幅值也越大，但无量纲振幅放大倍数随浮体横荡振幅的变化不明显；在某些激励频率下，浮体垂荡与结构耦合的动响应过程中会出现模态转换现象，表现为高阶模态向低阶模态转换，发生模态转换后，位移幅值会有明显增大。